1. Field of the Invention
This invention relates to a method and apparatus for heating flat or curved surfaces such as those found in paper, textile and food drying equipment. More particularly, this invention relates to a method and apparatus for heating flat or curved surfaces employing indirect firing means to increase the temperature uniformity and heat flux of equipment employing such surfaces, to reduce NOx emissions formation utilizing flue gas recirculation, and to increase the overall system thermal efficiency.
2. Description of Related Art
A significant amount of heating/drying, such as paper and textile drying, is carried out in indirectly fired equipment that is heated by steam. This approach limits the maximum heating surface temperatures because of the increasing steam pressure requirements at higher temperatures. As a result, equipment such as steam paper dryers are generally limited to temperatures below about 400° F. Several gas-fired heater/dryer concepts have been developed for providing higher heating surface temperatures, but implementations of these are generally complex in design.
One approach to addressing these issues has been the use of fuel combustion within a drum dryer to heat the surface of the drum, either through the use of radiant burners or open flame burners that direct combustion gases at the drum surface. U.S. Pat. No. 5,416,979 teaches a paper drying apparatus consisting of a rotatable dryer drum with a plurality of burners located in a hood partially surrounding the drum and means for directing hot combustion products towards the wet paper. U.S. Pat. No. 5,791,065 teaches a rotating drying cylinder with a gas-fired assembly employing segmented burners mounted in the interior of the cylinder to transmit heat to the cylinder by convection and infrared radiation. U.S. Pat. No. 5,842,285 teaches a drying apparatus having a number of independently controllable radiant gas burners located inside the drum to heat the drum. PCT International Publication WO 99/45196 teaches a rotating drying cylinder having a ribbon burner facing towards and adjacent to an inner surface of the cylinder and an acoustic baffle mounted behind the burner. In addition, a hollow roller heated by burning fuel internally and directing hot products of combustion against the inner surface of the roller and a baffle means disposed behind the burner for absorbing sound is also taught by this publication.